build: require node v20 (#7333)

Set minimum node version to v20 (from v18)

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7333
Reviewed-by: 0ko <0ko@noreply.codeberg.org>
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: Michael Kriese <michael.kriese@visualon.de>
Co-committed-by: Michael Kriese <michael.kriese@visualon.de>

package-lock.json

@@ -103,7 +103,7 @@
         "vitest": "3.0.8"
       },
       "engines": {
-        "node": ">= 18.0.0"
+        "node": ">= 20.0.0"
       }
     },
     "node_modules/@alloc/quick-lru": {

package.json

@@ -2,7 +2,7 @@
   "name": "forgejo",
   "type": "module",
   "engines": {
-    "node": ">= 18.0.0"
+    "node": ">= 20.0.0"
   },
   "dependencies": {
     "@citation-js/core": "0.7.14",